Mechanical Characteristics And Deformation Of Existing Metro Structures Due To Undercrossing Tunnel Construction

With rapid development of urban rail transits, more and more tunnels need to be constructed underneath existing subway structures. When a new tunnel traverses an existing tunnel, the construction will bring about varying degrees of strata disturbance, which results in loading and deforming to both the existing tunnel and the tracks in the tunnel. If the deformation exceeds its ultimate value, it will affect the safety of metro operation. Using finite element method, mechanical characteristics and deformation of existing metro due to traversing tunnel construction is studied by numerical simulation. The influence of factors, such as settlement of strata, depth of both existing tunnel and new tunnel, modulus of elasticity of the soil and the spacing of newly constructed tunnels, are discussed. The main results are as follows:1. The relationships of the new tunnel vault settlement with the deformation characteristic values of existing tunnel, such as the maximum settlement, the range of influence, the settlement slope, the geometric irregularity deviation of tracks in static, were set up. The research results showed that the settlements of groud surface and existing track become greater with the increase of the new tunnel vault settlement. When the vault settlement of new tunnel is up to 53 mm, the maximum settlement of the existing track is 13 mm, and the static longitudinal and alignment deviations of the track are 2.5mm/10m and 1.1 mm/10m, respectively. The track longitudinal maximum tensile stress and compressive stress are respectively 9.5MPa and 5.4MPa producted by new tunnel construction. Adding basic stress, the total stress of the track is 325.2MPa which satisfies the strength requirement of track structure. Through comprehensive analysis, it is suggested that the maximum settlement of the existing track structure should not be allowed to exceed 13mm, and the adjustment altitude of rail fastener can be as safety reserve.2. When the distance between new tunnel and existing tunnel is less than the tunnel diameter, the static longitudinal deviation of the track raises with the increase of the depth of whether new tunnel or existing tunnel, while the static alignment deviation of the track decreases. When the depth of existing tunnel increases 3m, the longitudinal tensile stresses of the existing track and segment increase 2MPa and 0.5MPa, respectively. When the depth of new tunnel increases 3m, the longitudinal compressive and tensile stresses of the existing segment increase 0.8MPa and 0.5MPa, respectively. However, there is a small influence in the longitudinal tensile stress of the track. The settlement of existing tunnel structure rises with the decrease of the elastic modulus of the strata, while there is no much effect on the horizontal deformation.3. When the spacing of new double-line tunnels is half a diameter of the tunnel, there is superimposed effect on existing tunnel, and the settlement trough widths of the strata and existing tunnel structure enlarge either. The larger the spacing of new double-line tunnels, the smaller the superimposed effect, bringing about larger settlement trough width and smaller settlement of the soil and existing structure. When the spacing of new double-line tunnels becomes 4 times diameter of the tunnel, there is a double-wave in the settlement trough of the intercalation soil between new tunnel and existing tunnel. However, within 15m on each sides of central axis, the settlement of track shows a flat curve. It is shown by analytical calculation that the settlements of the existing tunnel and track, the range of influence, the distance of inflection point and the settlement slope have linear relationships with the vault settlement of new tunnel, respectively.4. The static geometric irregularity deviation of the existing track increases with the decrease of the spacing between new double-line tunnels. When the spacing of new double-line tunnels is the half diameter of the tunnel, the maximum settlement of the existing track is 14.9mm, and the longitudinal and alignment static deviations of the track are respectively 2.6mm/10m and 2.4mm/10m, less than the allowable deviation 4mm/10m. Based on the stresses analyses on the existing tunnel structure, the ballast, the track and the intercalation soil between new tunnel and existing tunnel, considering the adverse influences on the ballast structure due to the superimposed effect under double-line tunnels construction, it is suggested that the spacing between new double-line tunnels should not be less than the tunnel diameter, and settlement of track structure should not be greater than 14.6mm, so that the safety of metro train operation can be satisfied. Meanwhile, the adjustment altitude of rail fastener can be as safety reserve.